High fidelity pulse multiplex system



2 Sheets-Sheet l I INVENTOR U fkeA/w /Q/ 1/0/ ATTORNEY Qmfll March 27, l951 B. A. TREvoR HIGH FIDELITY PULSE: MULTIPLEX SYSTEM Filed Sept. 28. 1945 March 27, 1951 B. A. TREVOR HIGH FIDELITY PULSE MULTIPLEX SYSTEM Filed Sept. 28. 1945 2 Sheets-Sheet 2 INVENTOR ffQAP/IM )22m/0x@ ATTORNEY N Nh time intervals between Patented Mar. 27, 1951 HIGH FDELITY PULSE MULTIPLEX SYSTEM Bertram A. Trevor, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation a of Delaware Application September 28, 1945, Serial No. 619,235 e claims. (ci. 17e- 15) This invention relates to multiplex communication systems, and particularly to such systems utilizing pulses of electrical energy for communication purposes.

It is known in multiplex systems to employ pulses of radio frequency energy which are short compared to the time intervals between them, and wherein the pulse outputs from the different channels are independently modulated for the transmission of independent programs or modulations. The different channels thus carry different messages, and these messages may be speech or telegraph signals. The modulation mayy be effected by modulating the amplitude, width (duration) or timing of the pulses. Such systems are sometimes referred to as time division multiplex systems because the common transmitting circuit (any suitable transmission medium) is assigned consecutively to successive channels 'fi'- receiving terminal) are also combined and a for equal time intervals.

The pulses in the common transmitting cirshort compared to the them. It is preferred, though not essential, that the pulses from all the channels occupy only a small percentage of the total time Vvfor all conditions of modulation, in order that the common transmitter can furnish a peak power which considerably exceeds the power obtainable in a continuous wave systern.

In such systems, the pulse repetition rate may,

for example, be 10,000 per second (l kc.) for each channel. Where an eight channel pulse multiplex system is employed utilizing a synchronizing pulse for each synchronizing period, the overall pulse rate from the system may be 90,000 per second (90 kc.). This number is obtained by assigning 10,000 pulses per second for each of the eight ordinary fidelity channels and an additional 10,000 pulses per second for synchronization purposes. Ordinarily the pulse rate is about three times the highest modulation frequency. Although a pulse repetition of l0 kc. for each channel is sufficient to satisfactorily transmit signals having an audio frequency of 3000 cycles and less, it will be evident that such a repetition rate is not satisfactory for transmitting speech waves having higher audio frequencies, for example 5,000 to 9,000 cycles.

An object of the present invention is to provide a pulse multiplex system having one or more high fidelity channels along witha number of ordinary fidelity channels.

cuit may or may not be Another object is to provide a multi-channelk bined channels radio system having at least one high telephone quality audio frequency circuit and one or more lower telephone quality audio frequency circuits.

These objects of the invention are achieved by combining two or more channels in the multiplex system so as to raise the effective pulse repetition rate and thereby permit higher audio frequencies to be carried with greater delity by the combined channels. As an illustration, assuming an eight channel pulse multiplex system with a pulse repetition rate of l0 kc. per channel, I contemplate combining the inputs of channels l, i and 'l at the transmitting terminal and supplying high quality audio frequency up to 9 kc. to all three comsimultaneously. In this way, an effective pulse frequency of kc. is available from the combined channels for carrying the modulation. In this particular illustration, the receiving multiplex channels l, li and 1 (at the suitable single 9 kc. lter utilized.

In this illustration of an embodiment of the invention, it is assumed that channels (channel circuits) i, 4 and 'l of an eight channel multiplex system are combined toprovide a single high fidelity channel. The modulation frequency band passed by this high fidelity channel is 9 kc. In such a system, channels (channel circuits) 2, 3, 5, 6 and 8 each provide ordinary fidelity channels and pass a 3 kc. modulation frequency band. Obviously, other combinations of channels can be used to achieve a high fidelity channel, such as combining channels 2, 5 and 3. The channel numbers selected for combination are preferably such that the pulses from these respective channels are evenly spaced apart in time.

If only two channels of a multiplex system are to be combined in accordance with the invention, it is then desirable to employ a multiplex system containing an odd number of channels so that there are an even number of time periods including the synchronizing signal. As an example, if the multiplex system is designed to be a seven channel system utilizing a synchronizing pulse for each cycle of operation or synchronization period, then channels l and 5 can be combined lto provide a high fidelity channel furnishing evenly spaced pulses at twice normal repetition rate (at twice the pulse repetition rate of any of the other channels) The invention is particularly useful in a radio multiplex system or in a radio relay system in which it is desired to transmit and receive high quality speech or audio signals along with'ordinary quality speech or audio signals.

The invention will now be described in more detail in connection with a particular type of eight channel pulse multiplex radio system which has been developed and successfully tried out by the Radio Corporation of America. It should be distinctly understood, however, that the principles of the invention are applicable to various types of multiplex or time division systems having diierent numbers of channels, and that the invention is not limited to the particular system described hereinafter which is given as an example only.

In the drawing:

Figs. l, la and 2 illustrate, diagrammatically, the transmitting and receiving ends, respectively, of complete pulse multiplex radio system in accoi-dance with the invention.

The transmitter or Fig. 1 is, in effect, a modification of the pulse multiplex system disclosed in copending application `Serial No. 608,957, rlled by William D. Houghton on August 4, 1945, now U. S. Patent 2,531,817, granted Nov. 28, 1950, to which reference is herein made for the electrical circuits oi the apparatus in the various boxes. Essentially, this modication comprises combining three channels to obtain a single high delity channel. The other ve channels are ordinary delity channels.

The transmitter of Fig. 1A will rst be described as having eight individual channels, and later an explanation will be given on how three of these channels are combined into a single high delity channel. This transmitter utilizes short pulses of radio frequency energy which are time displaced by modulation. For multiplexing purposes, the pulses corresponding to the separate eight channels are separately and consecutively generated at a fixed repetition rate which will be called hereinafter the synchronization rate, correspondingr to a fixed time interval to be called the synchronization period. In this transmitting system, the synchronization rate is kc. and the corresponding period 100 aseo. (microseconds). A pulse occurs once each synchronization period for each of the eight channels. rates and periods are consecutively the same and equal to the synchronization rate and period. Each channel pulse occurs at a rate of l kc. and the separation between adjacent pulses in each channel for an unmodulated condition is 100 i" aseo., or microseconds. Because the unmodulated signal pulses are similarly located in each channel, they are, therefore, about 11 microseconds apart in the common output circuit. rhe pulses in each channel can be modulated i4 aseo. (peak i modulation), thus leaving a guard space between pulses from succeeding channels of about 3.1 aseo. The guard space is necessary to reduce cross modulation effects. The pulses from other channels occur in the interval between adjacent pulses from any one channel. The synchronization pulses occupy the ninth interval of 11.1 microseconds. The output pulses from the channel are of constant length and the time between two adjacent pulses is measured from the leading edges. The pulses from the channels are equally spaced in the absence of modulation.

In Fig. l there is shown a 90 kc. crystal oscillator l0 producing pulses of current which feed into and lock by injection a 90 kc. short pulse oscillator I. The short output pulses from the pulse oscillator I2 are applied to a counter or vstep-wave generator i4. The voltage wave forms from the apparatus i6, I2 and id are illustrated The individual if.,

by the curves 9, II and I3, respectively, appearing immediately above the equipment.

The counter I4 provides two outputs, one of which is the step wave I3 which is supplied to the coupling amplier IS and the other of which is a synchronization pulse I3 occurring once for each step wave cycle and which is applied via lead I5 to a synchronization pulse generator 30. The function of the step wave I3, which is applied to the coupling amplier and then to the different channels over lead I?, is to time the mean occurrence of reach channel pulse. The output of the coupling ampliner I6 is applied to a connection I9 which is common to all the channels i to B, inclusive.

All channels are substantially identical; and each includes in the order named, a channel selector I8, normally non-conductive saw-tooth generator 2E! controlled by the output of the channel selector I 8, a pulse generator 22, and an output circuit including a Shaper or clipperlimiter 24. The pulses produced by the channel pulse generator 22 are modulated as to time ror phase by means of a modulator 25 which is supplied with suitable signals, such as speech, from an audio amplifier 28. All channels have their channel selector inputs connected together in electrically parallel relation.

The channel selectors are differently self-biased and each channel selector is normally biased well beyond the current cut-off condition. The bias of each channel selector is so adjusted that the applied step wave from the coupling amplier I6 causes current to flow consecutively in the different channel selectors. One channel selector conducts for each rise of voltage in the step wave I 3 up to eight, which is the number of channels. Each step rise in the step wave is great enough to insure that during its occurrence the current of the correspondingly biased channel selector shall be driven rapidly from beyond the cut-off condition to a zero bias value. Once a channel selector starts to conduct, thev current iow therein will continue until the end of vthe synchronization period, when the input voltage to the selector drops to zero at the end of the step. The outputs from all channels appearing in leads 29 iiow in a common lead 3i to dilerentiator and clipper circuits 32 and 34 from which pulses of shorter duration are applied to a power amplier 36 whose output controls the production of radio frequency pulses from a, magnetron oscillator 40. The very short duration output pulses from magnetron 4i) which may each have an eiective duration of 0.3 aseo., are fed to antenna 42, from which they are radiated t0 the remotely located receiving terminal shown in Fig. 2.

The synchronization pulse generator 3) which receives a pulse over lead I5 from the counter i4 at the end of each step wave, produces a pulse at the end of each step wave which is supplied to the differentiator and clipper 34 and also fed to the ampliiier 36 and magnetron 4I! together with the channel pulses. There will thus be eight consecutively appearing pulses from the eight different channels followed by a synchronization pulse for each cycle of operations. For the unmodulated condition, all of these channel pulses and the synchronization pulse will be separated from one another by a spacing equivalent to 11.1 aseo. It will thus be seen that the synchronization period of psec. is divided into nine equal intervals by the step counter of step wave 'generator I4, and that lall of these pulses are similarly located in each one of these nine equal intervals and similarly spaced apart for the unmodulated signal condition.

In order to provide a single high idelity channel capable of carrying higher telephone quality audio frequencies than the other channels, the inputs of audio frequency ampliers 2S in channels I, i and l are combined and fed with the same high quality audio signal. This signal can be speech or other suitable signals ranging up to 9 kc. The other channels, namely 2, 3, 5, 6 and 3, are individually modulated by separate audio signals ranging in frequency up to about 3 kc. It will thus be seen that the effective pulse repetition rate of the three combined channels forming a single high iidelity channel is 30 kc., while the pulse rate for each oi the other channels carrying different modulation is i kc.

Fig. lo. graphically illustrates the approximate appearance of theenvelope of the time displaced pulses in the common output of magnetron oscillator Gil, as impressed on transmitting antenna ft2, for the unniodulated condition. In Fig. la there are eight short channel pulses and one longer synchronizing pulse for each synchronizing period. This is because the pulse produced by the synchronizing pulse generator 3B is of longer duration than the individual channel pulses and periodically occurs after every eighth channel pulse. Fig. lc also shows the appearance of the pulses the output of amplifier' 36. Amplifier 35 supplies direct current pulses to the 'cathode of the magnetron :le and causes the magnetron to produce radio frequency pulses of a duration somewhat less than the duration of the pulse appliefJ to the magnetron. rI'he radio frequency pulses from the magnetron fill may have a frequency of i356 to 1459 rnegacycles, by way oi example, and have a duration of about .4 sec. for the channel pulses and 2.0 csec. for the synchronizing pulse.

Because the signal inputs of channels I, and .l are combined, as shown in Fig. l, the same modulation will appear on lthe pulses designated I, fi and 'I of Fig. la, whereas independent modulations will appear on pulses .2, 3, 5, il and 3 representing the other channels. It should be noted that the pulses of combined channels i, Il and 'i' representing the single high iidelity channel are spaced apart by two other pulses. Thus, the pulse in channel I is spaced apart -rom the pulse in channel d by the two pulses of channels 2 and 3, and similarly the pulse in channel I is spaced apart by the pulse in channel l from the .two pulses or channels and 5. Also, the pulse in `channel l spaced apart from the pulse in channel I oi the next synchronizing period by the channel pulse of channel and also by the synchronizing pulse.

Fig. 2 illustrates the receiving terminal of the ulse multiplex system for receiving the pulses sent out by the transmitter of Fig. 1 and for reproducing the original modulations. In Fig. 2, the pulse outputs of the three channels I, 4 and combined and fed to a single 9 lic. low pass iilter, whereas the pulse outputs of the other channels 2, 5, and 8 are individually fed to separate 3 kc. low pass iilters for reproducing the individual audio signals.

Fig. 2 includes an antenna 5d for receiving the radio signals from the remotely located transmitter of Fig. l, and a superheterodyne receiver 52 upon which the incoming pulses of radio frequency energy are impressed from the antenna. The' output ofthe receiver 52 at lead 54 is in the 6'. form of spaced video (direct current) pulses. These video pulses then follow two paths, one path extending to. apparatus 56 shown as a box in dash lines, andthe other path indicated as lead 5S extending to the gates of all the channels.

Apparatus 56 separates the synchronization pulses from the channel pulses in the output of the superheterodyne receiver 52 and produces from these separated synchronization pulses a new step wave available at lead 63 whose amplitude is adjustable and whose phase is adjustable relative to the incoming pulses. This new step wave produced by apparatus 5E is similar to the step wave (note graph I3, Fig. l) at the transmitter of Fig. l but is independent of the modulations in the channels. Each rise in this new step wave has ardiierent amplitude and controls a different channel selector in the channels of Fig. 2 in substantially the same manner as described above in connection with Fig.y i.

Apparatus 5% includes a synchronizing pulse separator il which distinguishes between the longer duration synchronizing pulses and the shorter channel pulses. k Putting it in other words, separator circuit il can be called a pulse selective system which enables the utilization of only the synchronization pulses.. The output of separator circuit synchronisation p riod (190 micro-seconds). One suitable circuit which can be used i'or separator is shown in Fig. 2a oi copending application Serial No. 515,352, led by William D. Houghton on September l0, 1945, now Patent No. 2,532,843, granted December 5, 1950, to which reference is made.

The output of synchronization pulse separator :il is passed through a coupling amplifier t9 and then to phasing trigger 5I which produces pulses of adjustable phase at the rate ci one pulse for each 1GO microseconds. r)The output of phasing trigger lliis usedto excitea lic. exciter 53 which. may bein the form of a tuned circuit. In effect, exciter is a'harinonic generator which produces a sine wave output of 90 kc. frequency. This 9u kc. sine wave output from box 53 is fed into a 90 kc. limiter 55 which produces a pulse for each peak of sine wave iinpressed thereon. Thus, the output from limiter 5E comprises 9() Kc. pulses. IThese 90 kc. pulses from limiter 55 are i'ed to a step wave generator or pulse counter 5l which in turn produces a step wave voltage having nine steps or risers. Such a pulse counter may be similar to the one used at the transmitter and described in more detail in copending application Serial No. 608,957, supra.

The step wavevoltage in lead 6u (output from apparatus 55) is fed to the inputs or" the different channel selectors I8. Each channel selector I8 is normally non-conductive and passes current at a particular' rise or amplitude in the step wave voltage, dependent upon previous adjustments. The different channel selectors I8', lilre those in the transmitter of Fig. 1, are biased differently, and the bias is so adjusted that the applied step wave in lead E@ causes current to flow consecutively in the different channel selectors. One channel selector I3 conducts for each. rise of voltage in the step wave up to eight, and each rise or step of voltage is great enough to insure that during its occurrence the current of the correspondingly biased channel selector vshall be driven rapidly from beyond the cut-off condition to a zero bias value.

is a single pulse for each The passing of current by a channel selector I8 causes the tripping of its associated trigger circuit 62. Trigger circuit 62 is a flip-flop or unbalanced circuit having one degree of electrical stability. Such trigger circuits are known in the art and may consist of a pair of vacuum tube electrode structures whose grids and anodes are interconnected regeneratively. The trigger circuit has a stable state and an active state. A tripping pulse of suitable polarity serves to trigger oli or trip the trigger circuit from its stable to its active state. In the stable state, one electrode structure passes current and the other elect-rode structure is non-conductive. lThese current passing conditions of the two electrode structures are reversed in the active state.

After the trigger circuit is tripped into the active state, it is restored to its normal or stable state by the gate Sli which is responsive to the video pulse in lead 58 which immediately follows in time the riser at which the channel selector started to conduct. Although the trigger circuit is self-restoring in character, it is given such a time constant that once tripped into the active state, it remains in this active state for a time interval sufficiently long to extend beyond the time in which its channel pulse is expected to arrive. The video pulses in lead 58 are impressed on .gates 64 in the diierent channels, and these gates control the trigger circuits in response to the proper channel pulses. It should be understood that the leading edge of the channel pulse is used to control the trigger circuit 62, whereas the trailing edge of the synchronizing pulse is used in the apparatus S. The modulation could be taken from either the leading or trailing edges. The output of each trigger circuit $2 is a variable width constant amplitude pulse whose time duration depends upon the time of arrival of the channel pulse. Thus, it will be seen that the time displaced incoming (received) pulses of constant duration have been converted into variable width pulses whose duration (width) corresponds with the time displacement of the incoming pulses. Stated in other words, the incoming pulses of variable occurrence time have been changed to pulses of variable width having the same modulation.

The outputs of the triggers G2 of channels i, i and 'l are combined and fed via lead 65 to the 9 kc. low pass ilter 58. There is thus passed to lter B8 pulses at an eifective 3G lic. rate from three combined channels upon which the same audio frequency signal is impressed. Output from filter $3 is fed to an audio amplifier 'lil and then to a suitable transducer such as phones or a loudspeaker.

The outputs of the triggers 62 of channels 2, 3, 5, f5 and S are individually fed to separate 3 kc. low pass filters 'i2 and then to individual audio amplifiers and transducers for reproducing independent signals.

From the foregoing, it will be seen that l am able to obtain a vhigh delity channel from a conventional or known type of pulse multiplex system without making any major changes in the equipment. By combining other channels, it is possible to obtain two or more high delity channels along with a number of ordinary delity channels. f

yWhat is claimed is:

1. In a pulse multiplex system having a multipl'ic'it'y of. independent channels, a transmitter having means for producing pulses and modulating a characteristic thereof in each of said channels, the pulses in the vdifferent channels having the same repetition rate, and means for supplying the same modulation to a plurality of said channels and different modulations to other channels, to thereby effectively increase the effective pulse frequency available to carry the modulation supplied to said plurality of channels, means combining ther outputs from all of said channels, means for causing said plurality of channels to produce pulses at such selected time intervals that the resulting combination of pulses from said plurality of channels is a series of regularly recurring pulses spaced evenly in time with pulses from the other channels occurring in the intervals between adjacent pulses in said series, a receiver having the same numberof channels as said transmitter, means at said receiver for combining the pulse outputs from a plurality of channels corresponding in number 1position to said plurality at said transmitter, a filter at the receiver for passing the combined outputs from said plurality, and other lters at the receiver of narrower frequency bands than said first filter for passing the pulse outputs of the other channels.

2. In a pulse multiplex system having a multiplicity of independent channels, a transmitter having means for producing pulses and modulating the timing thereof in each of said channels, the pulses in the dierent channels having the same repetition rate, and means for supplying the same modulation to a plurality of said channels in the same phase and different modulations to other channels, to thereby effectively increase the effective pulse frequency available to carry the modulation supplied to channels, said plurality of means combining the outputs from all of said channels, means for causing said plurality of channels to produce pulses at such selected time intervals that the resulting combination of pulses from said plurality of channels is a series of regularly recurring pulses spaced evenly in time with pulses from the other channels occurring in the intervals between adjacent pulses in said series, a receiver having the same number of channels said transmitter, means at said receiver for combining the pulse outputs from a plurality of channels corresponding in number and position to said plurality at said transmitter, a filter at the receiver for passing the combined outputs from said plurality, and other ters at the receiver of narrower frequency bands than said first ilter for passing the pulse outputs of the other channels.

3. A multiplex transmitting system having a plurality or" channel circuits, a common transmission medium, means Vfor assigning said medium consecutively to successive channel circuits for equal time intervals, means in each of said channel circuits for producing pulses, means for supplying the same modulation to at least two of said channel circuits which are non-adjacent in point of time and produce pulses spaced from each other and recurring regularly in time sequence, and means for supplying a different modulation to another channel circuit which is positioned between said first two channel circuits in point of time and produces pulses in the intervals between the pulses produced by said rst two channel circuits.

BERTRAM A. TREVOR.

(References on following page) REFERENCES CITED The following references are of record in the le of this patent:y

UNITED STATES PATENTS f Number Name Date 2,199,634 Koch May 7, 1940 2,277,192 Wilson Mar. 24, 1942 2,405,252 Goldsmith Aug. 6, 1946 Number Number Name Date Schroeder Aug. 20, 1946 Farrington Dec. 31, 1946 Labin Jan. 31, 1950 FOREIGN PATENTS Country Date Great Britain Mar. 12, 1942 

